save() breaks my pico - I have to do a firmware flash...

What code are you sending to it before saving that causes problems?

I'll update the troubleshooting page today, but the easiest way to recover is to plug in and then hold down the button (as soon after plugging in as you can). If done right, it won't enter the bootloader mode (with the LEDs pulsing) but will just go into Espruino without loading the saved code (you should then be able to type save() to save 'blank' state back to the Espruino).